Ambient radiation interference is a problem in many electronic imaging systems. In such systems, an image is formed on an image sensor by objective optics, which collect and focus optical radiation in a given wavelength range onto the image sensor plane. (Optical radiation, in the context of the present patent application and in the claims, may comprise any or all of visible, ultraviolet and infrared radiation.) In many applications, particularly for computer vision, the image is formed at a particular target wavelength. For example, in certain 3D imaging systems, patterned or pulsed light in a certain narrow band (typically in the infrared range) is projected onto a scene, and a camera captures an image of the pattern on the scene. In such applications, ambient background radiation collected from the scene at other wavelengths within the range of the objective optics tends to introduce spurious features and reduce the contrast (and hence reduce the quality) of the image captured by the image sensor.
In some applications of this sort, an interference filter is used in conjunction with the objective optics in order to permit optical radiation to reach the image sensor in only a narrow band of wavelengths, while rejecting radiation outside the band. One system of this sort is described, for example, in U.S. Patent Application Publication 2008/0240502, whose disclosure is incorporated herein by reference.